1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to an apparatus for driving a lamp of a liquid crystal display.
2. Description of the Related Art
In general, a liquid crystal display (hereinafter, referred to as a “LCD”) device has applications that are broadening due to its advantageous characteristics, such as lightweight, thin profile and low power consumption. The LCD device is typically used in office automation equipment, audio/video devices and the like. The LCD device displays a desired picture on a screen by controlling the amount of light transmitted through the device in accordance with a video signal applied via a plurality of control switches, which are arranged in a matrix form.
The LCD device needs a light source, such as a backlight, because the LCD device is not a light-emitting display device. A cold cathode fluorescent lamp (hereinafter, referred to as a “CCFL”) is generally used as the light source in the backlight unit. The CCFL uses a cold emission phenomenon (i.e. electrons are emitted due to a strong electric field applied to the surface of a cathode) to provide light with a high brightness, a long life span and a full colorization. The CCFL has low heat generation. The CCFL can be used in different light guide systems, such as a direct illumination system or a reflection plate system. The type of light guide system adopted for a specific LCD device is based on the physical requirements of the LCD device.
The CCFL uses an inverter circuit to get a high voltage from a low voltage DC power source to drive the CCFL. FIG. 1 is a block diagram illustrating an apparatus for driving lamps of a liquid crystal display device according to the related art. FIG. 2 is a schematic block diagram illustrating the apparatus for driving the lamps of the liquid crystal display device shown in FIG. 1.
Referring to FIGS. 1 and 2, an apparatus for driving lamps of an LCD device according to the related art includes a lamp housing 10 having a plurality of lamps 12, an inverter part 4 with a plurality of inverters for supplying an output voltage to each of the lamps 12, a first printed circuit board 2 on which the inverter part 4 is mounted, and a second printed circuit board 6 on which the lamps 12 are commonly connected to the ground voltage source GND. The lamp housing 10 has a space provided for receiving the lamps. The lamp housing 10 is stacked onto a main support (not shown). Each lamp 12 receives a lamp output voltage from the inverter part 4 and emits visible light to a liquid crystal display panel (not shown).
Each of the lamps 12 includes a glass tube with an inert gas inside of the glass tube. One side of the lamp 12 is connected to a secondary winding wire T2 of a transformer 16, and another side of the lamp 12 is connected to the ground voltage source GND. The inside of the glass tube contains inert gas, such as Ar or Ne, as well as phosphorus spread over the inner wall of the glass tube. When a high AC voltage supplied from the inverter 20 is applied to an electrode of one of the lamps 12, electrons are emitted that collide with the inert gas inside the glass tube, thereby increasing the amount of electrons by geometric progression. The increased electrons cause electric current to flow inside of the glass tube, thereby exciting the inert gas to emit ultraviolet light. The ultraviolet light irradiates phosphorus, which is spread over the inner wall of the glass tube, to cause the emission of visible light.
The first printed circuit board 2 is arranged at one side of the main support (not shown). The second printed circuit board 6 is arranged at one side of the main support (not shown). Each inverter 8 included in the inverter part 4 of the first printed circuit board 2 includes a switch device 14 to switch a voltage from a voltage source Vin in response to a switching control signal, a transformer 16 to convert the voltage supplied by switching of the switch device 14 into an output voltage, a voltage detector 20 to detect the voltage of the inverter 8, and a controller (Pulse Width Modulation: PWM) 18 to switch the switch device 14 in response to a feedback voltage FB from the voltage detector 20. The switch device 14 switches the voltage from the voltage source Vin to the transformer 16 in response to the switching control signal from the controller 18.
The transformer 16 includes a primary winding wire T1 connected to the switch device 14 and a secondary winding wire T2 connected to the lamp 12. Both ends of the primary winding wire T1 are connected to the switch device 14 and one side of the secondary winding wire T2 is connected to one side of the lamp 12, and at the same time, the other end of the secondary winding wire T2 is connected to the voltage detector 20. The transformer 16 converts the voltage supplied to the primary winding wire T1 to an output voltage on the secondary winding wire T2 in a ratio corresponding to a winding wire ratio of primary and secondary winding wires T1 and T2. The output voltage induced onto the secondary winding wire T2 is supplied to the lamp 12 through one side of the lamp 12, thereby turning on the lamp 12.
The voltage detector 20 detects the output voltage or high AC voltage induced onto the secondary winding wire T2 of the transformer 16 to generate a feedback voltage FB. In the alternative, the voltage detector 20 may be located at the output terminal of the lamp 12 to detect the output value of the voltage outputted from the lamp 12. The controller 18 receives the feedback voltage FB generated from the voltage detector 20 to control a switching period of the switch device 14. In other words, when the feedback voltage FB is higher than a reference voltage for driving the lamp, the controller 18 reduces a width of the switching control signal supplied to the switch device 14 to make a switching time of the switch device 14 fast. Because of this, the voltage supplied from the voltage source Vin to the transformer 16 is reduced so that a current passing through the lamp 12 is reduced.
On the other hand, when the feedback voltage FB is lower than the reference voltage, the controller 18 increases the width of the switching control signal supplied to the switch device 14 to make the switching period of the switch device 14 slow. Because of this, the voltage supplied form the voltage source Vin to the transformer 16 increases so that the current passing through the lamp 12 increases. Accordingly, the voltage supplied to each lamp 12 is constantly maintained so that the brightness of the light generated from the lamps 12 is constantly maintained.
When the temperature decreases in the apparatus for driving the lamp of the liquid crystal display device according to the related art, the brightness of the light generated from the lamp 12 is reduced. FIG. 3 is a graph showing a tube current of the lamp over time for different temperatures while driving the lamp of the liquid crystal display device shown in FIG. 2. When the temperature decreases, gas movement of the gas charged in the lamp 12 reduces so as to increase the resistance of the lamp 12. Because of this, the supply voltage monitored by the voltage detector 20 connected to the other end of the secondary winding wire T2 in the transformer 16 increases so that the feedback voltage FB increases. Accordingly, the controller 18 makes the switching period of the switch device 14 fast, thereby reducing the voltage supplied from the voltage source Vin to the transformer 16. Thus, as shown in FIG. 3, the current passing through the lamp 12 is reduced. This causes a problem in that the brightness of the light generated from the lamp 12 decreases.